FIG. 1 depicts a wireless communications system 10 employing Code Division Multiple Access (CDMA) techniques based on the well-known IS-95 standard of the Telecommunication Industrial Association. The wireless communications system 10 comprises a mobile switching center (MSC) 12 and a plurality of base stations (BS) 14-i connected to the MSC 12. Each of BS 14-i provides wireless communications services to mobile-telephones (MT), such as mobile-telephones 16-k, within an associated geographical coverage area referred to herein as cell 18-i with a radius R.sub.i. For illustrative purposes, cells 18-i are depicted as circular in shape with base stations 14-i centrally positioned. It should be understood that cells 18-i may also be non-circular in shape (e.g., hexagonal) with the base stations positioned non-centrally, and that the term "radius R.sub.i " should be construed to define a distance between the base station and a point on the circumference of cell 18-i (which will vary depending on the particular point on the circumference).
Each base station 14-i includes radios and antennas for modulating and transmitting base station signals to mobile-telephones, and for receiving and demodulating mobile-telephone signals from mobile-telephones within its associated cell 18-i. Each base station 14-i further includes a receiver for receiving timing information using the well-known Global Positioning Satellites (hereinafter referred as a "GPS receiver").
Signals are transmitted by base stations 14-i and mobile-telephones in accordance with a timing protocol aligned with GPS time using the GPS receiver. FIG. 2 depicts a timing schedule 20 incorporating an implementation of a timing protocol based on the IS-95 standard. The timing schedule 20 comprises a series of frames 22-n, wherein each frame 22-n spans a time interval t. The beginning of each frame 22-n is marked by a frame boundary at time T.sub.n aligned to GPS time. In accordance with the timing protocol, base stations 14-i are configured to begin transmitting base station signals at the frame boundaries, wherein the base station signals include zero or more information bearing signals and a pilot signal for coherent demodulation of the information bearing signals by the mobile-telephones and system access operations. By contrast, mobile-telephones 16-k are configured to begin transmitting mobile-telephones signals at some multiple x of a frame time period (i.e., tx) after mobile-telephones 16-k began receiving base station signals, where x is some integer greater than or equal to zero. Unlike base station signals, mobile-telephone signals include one or more information bearing signals and no pilot signal, and are encoded using a set of orthogonal codes (referred to as Walsh codes) combined with a pseudo-noise (PN) sequence (or a known code) such that the information bearing signal may be non-coherently demodulated. The PN sequence comprises random 0 and 1 digital signals, wherein the duration for a 0 or 1 to transmit is referred to herein as a PN chip.
The above described timing protocol will now be discussed in reference to FIG. 3, which depicts a time chart 28 illustrating a sequence of transmissions and receptions by base station 14-i and mobile-telephone 16-k. At time T.sub.1, BS 14-i begins transmitting base station signal S.sub.1 to MT 16-k, which may be located anywhere in cell 18-i. MT 16-k begins receiving signal S.sub.1 at time T.sub.1 +d.sub.BS.fwdarw.MT, where d.sub.BS.fwdarw.MT is a propagation delay from BS 14-i to MT 16-k. Note that the term propagation delay shall be construed to include line-of-sight and non-line-of-sight propagation delays.
MT 16-k will wait a time interval tx from when MT 16-k began receiving signal S.sub.1 before it begins transmitting mobile-telephone signal S.sub.2. Thus, MT 16-k will begin transmitting signal S.sub.2 at time T.sub.1 +d.sub.BS.fwdarw.MT +tx (or time d.sub.BS.fwdarw.MT after some frame boundary). For example, if x=2, then MT 16-k transmits signal S.sub.2 at time T.sub.3 +d.sub.BS.fwdarw.MT (or two frames after receiving the base station signal S.sub.1).
Due to a propagation delay d.sub.MT.fwdarw.BS from MT 16-k to BS 14-i, BS 14-i will begin receiving signal S.sub.2 at time T.sub.1 +d.sub.BS.fwdarw.MT +tx+d.sub.MT.fwdarw.BS. For ease of discussion, it is assumed that the propagation delay d.sub.MT.fwdarw.BS from MT 16-k to BS 14-i is the same as the propagation delay d.sub.BS.fwdarw.MT, and both will hereinafter be referred to individually as a one way propagation delay d.sub.ow, i.e., d.sub.ow =d.sub.MT.fwdarw.BS =d.sub.BS.fwdarw.MT, or collectively as a round trip propagation delay 2d.sub.ow. Thus, BS 14-i will begin receiving signal S.sub.2 at time T.sub.1 +tx+2d.sub.ow.
In order to demodulate the received signal S.sub.2, BS 14-i must first detect signal S.sub.2. Each radio includes a correlator, which is a device that detects mobile-telephone signals. For example, the correlator detects mobile-telephone signal S.sub.2 by multiplying an incoming signal by the PN sequence, where the PN sequence is time shifted in discrete steps over a period or time interval (referred to herein as a search window W.sub.n) until the resulting product (of the PN sequence and the incoming signal) exceeds a threshold indicating the detection of mobile-telephone signal S.sub.2. If BS 14-i does not begin to receive signal S.sub.2 within the confines of a search window W.sub.n, BS 14-i will not be able to detect signal S.sub.2 (using the timing protocol incorporated in FIG. 2).
To ensure that BS 14-i begins receiving signal S.sub.2 within the confines of search windows W.sub.n, search windows W.sub.n should span time intervals that include possible arrival times for signal S.sub.2 (traveling a straight line or line-of-sight path between the mobile-telephone and the base station) regardless of the position of mobile-telephone 16-k in cell 18-i. Based on the above described timing protocol, base station 14-i can expect to receive signal S.sub.2 no earlier than the frame boundary and no later than time 2d.sub.ow-radius after the frame boundary, where d.sub.ow-radius is the one way propagation delay (or 2d.sub.ow-radius is the round trip propagation delay) for a signal traveling a distance equal to the radius R.sub.i. Thus, search windows W.sub.n should span a duration of at least 2d.sub.ow-radius beginning at time T.sub.n and ending no earlier than time T.sub.n +2d.sub.ow-radius. In effect, the duration of search windows W.sub.n restricts the effective radius (or size) of cell 18-i, which is also referred to herein as the access range of a base station.
The duration of search windows W.sub.n depends on the implementation of the correlator. Typically, correlators are implemented in the form of an Application Specific Integrated Circuit (hereinafter referred to as an "ASIC correlator") having a predetermined number of bits (also referred to herein as a "bit limitation") for representing a round trip delay (of a signal traveling from the base station to the mobile-telephone and back to the base station). Such bit limitation limits the duration of the search windows which, as discussed above, limits the effective size of cell 18-i or access range of the base station 14-i. As long as the bit limitation does not limit search windows W.sub.n to a duration of less than 2d.sub.ow-radius, base station 14-i should be able to detect signal S.sub.2 transmitted by any mobile-telephone located anywhere within its cell 18-i (assuming that R.sub.i is the same for all points on the circumference).
Typical implementations of base stations in an IS-95 based CDMA wireless communications system include an ASIC correlator having a 12-bit limitation for representing the round trip delay. In order to have fine resolution of delay, a typical value of 1/8 PN chip is used as the minimum resolution unit. The 12-bit limitation (or round trip delay representation) in units of 1/8 PN chips yields a range of 512 PN chips (i.e., 2.sup.12 bits.times.1/8 PN chips/bits). For a transmit bandwidth of 1.2288 MHz (which is typical for an IS-95 based CDMA wireless communications system), the 12-bit limitation can represent a round trip delay of 416 .mu.s (i.e., 512 PN chips.div.1.2288 PN chips/.mu.s). With air propagation speed of 5.33 .mu.s/mile, the 416 .mu.s round trip delay (or 208 .mu.s one way delay) represents the limitation that if a mobile-telephone is located approximately 39 miles (i.e., 208 .mu.s.div.5.33 .mu.s/mile) from the base station, the mobile-telephone is capable of communicating with the base station if the radio path loss is acceptable and the search window is configured correctly--that is, the 12-bit limitation (or 512 time chip delay index representation) allows for a cell with a maximum radius R.sub.i (or a maximum round trip delay) of approximately 39 miles. A signal transmitted by a mobile-telephone beyond 39 miles of BS 14-i, in accordance with the prior art timing protocol, may not arrive at BS 14-i within the confines of any search windows W.sub.n and, thus, will not be reliably detectable with the 12-bit ASIC correlator.
Presently, if the cell size or access range is to be extended beyond the 12-bit limitation of the ASIC correlator (i.e., beyond 39 miles), the ASIC correlator would have to be re-designed. Specifically, the ASIC correlator would have to be re-designed to increase its bit limitation such that signals transmitted by mobile-telephones positioned beyond the access range 12-bit limitation of the ASIC correlator may also be detected. ASIC correlator re-design, however, is undesirable and may not be economical for small scale of applications. Therefore, there exist a need to extend the cell size or access range of the base station without incurring the high costs associated with ASIC correlator re-design.